Where did you install them? A "personal" or "C3D" collection, perhaps? Or do they self install?
What I always do is dowload the zip, and not a self installing version of the items, so I can install them where I want.
If they self install, I bet they are in the appropriate folder, for example, if they are trees, you'll find them in the trees collections of your plants, if they are buildings, you should find them in the building folder of your objects collections, maybe in a subfolder called "sfi fi" or "monuments", since many objects sub categories have been added to the collections structures to match those of the store.